ELM STEM 63 



lignified : intercellular spaces may be noted here also, similar 

 to those above described as occurring in the medullary rays of 

 the wood. The cells contain copious protoplasm. 



2. Phloem-parenchyma, the cells of which differ in their cell 

 contents 



a. Some contain crystals. 



b. Others have copious protoplasmic contents. 



Both forms will be seen to have been derived by division 

 from original elongated cells with pointed ends, since they are 

 arranged in groups of this form. Compare the form of the 

 cambium cell as described below (IX.). 



3. Sieve-tubes answering to the description given for radial 

 sections. The sieves are oblique, the form of the successive 

 segments oblong. The sieves are often callous, and are easily 

 recognized in sections stained with iodine or eosin. 



4. Bast- fibres as before in radial sections. 



IX. Cut tangential sections through the cambium of the 

 stem of Elm : treat with dilute potash or " eau de javelle," and 

 mount in glycerine. Examine first with a low power, and note 

 that the general arrangement is similar to that already seen in 

 tangential sections through the mature tissues, also that the 

 form of the cells, in each part of the cambium-zone, is like or 

 similar to the average form of the elements of the mature 

 portion of wood or bast, which borders on it in a radial 

 direction. Thus the cambium is differentiated into 



1. Cambium, of medullary rays, which appears to consist of 

 roundish cells, resembling cells of the mature medullary rays in 

 form. 



2. Cambium, from which all the other tissues are derived, 

 the cells of which have a prismatic form. 



To gain a clear idea of the process of secondary thickening 

 the actual form of the cambium-cells and their arrangement 

 must be recognised : as stated above (p. 6), it is necessary, in 

 order to fully realise the form of a cell as a solid body, to cut 

 sections in three directions at right angles to one another : the 

 cambium-cells have now been seen in transverse, radial, and 

 tangential sections, and the results are represented diagram- 

 matically in Fig. 7, A, B, C, which are based upon results of 



